1. Field of the Invention
This invention relates to linear elements that are intended to extend along overhead electrical power transmission systems, and especially, although not necessarily, to optical cables that are supported along the route of the system by means of towers, masts or other upstanding supports that are also employed to support electrical power cables.
2. Brief Discussion of the Prior Art
In systems of this kind it is the general practice to earth the optical cable or cables at the towers, masts or other supports (hereinafter simply referred to as towers). When the electrical power lines are on load, electrical currents may be capacitively induced on the optical cable due to the distributed capacitance between the cable and the power lines. The voltage induced on the optical cable will reach a maximum at mid span between the towers, while the current flowing along the cable will be greatest in the region of the towers. Under dry conditions the induced currents will be relatively small due to the relatively high longitudinal resistance of the cable, e.g. in the region of 10.sup.12 Mohm m.sup.-1 but under wet conditions when the surface resistance of the cable is much lower, e.g. in the region of 10 Mohm m.sup.-1 much higher currents will be induced. Joule heating of the cable surface by the induced currents can cause a short length of the cable surface to become dry, usually in the region of a tower where the current is highest. When this happens the major part of the induced voltage on the cable is dropped across the short dry band due to its high longitudinal resistance, and so called "dry-band arcing" may occur which can cause severe damage to the cable. Similar problems may be experienced with other linear elements that may be extended between towers of the transmission system, for example with ropes that are used to install optical cables in the power transmission system. Although such ropes are only intended to be temporarily suspended between the towers, they can absorb significant quantities of water in wet ambient conditions, with the result that damaging currents may be induced.
It is possible to overcome the problem of dry-band arcing in an optical cable by providing the cable with a longitudinally extending electrically conductive path. However, an optical cable having such an electrically conductive path has the disadvantage that there are considerable safety issues to be taken into account if it is to be installed between towers of an overhead electric power transmissions line that is on load in view of the danger of it touching one of the transmission lines; moreover, it is not always possible or desirable to interrupt the electrical power transmitted by the overhead electric power transmission line for a time sufficient to enable such an optical cable to be installed.
It has been proposed, for example in European Patent Application No: 214,480, to employ a cable having a semiconducting element and a linear resistance of 10 to 10.sup.6 Mohm m.sup.-1. However, it is normally very difficult to form semiconducting elements having such resistance values in a consistent and reproducible manner using conventional methods since the particular resistance value is extremely sensitive to filler loading etc. with the result that nominally identical cables may have significantly differing resistances, and that the resistance of a cable may vary significantly along its length. Furthermore, we have found that such values of linear resistance are undesirably high since the linear conductivity of such a cable can be determined by pollutants--e.g. acids and salt--found on the cable in wet conditions.